In an oil or gas production well, the fluid path from the formation to the casing has traditionally flown directly into the casing from the rock (barefoot completion) or through perforations in the casing wall. More recently, slotted liners, sliding sleeves or screens have been introduced, all creating a simple flow path into the casing.
These solutions have traditionally relied on the flow being generated by creating a differential pressure from the reservoir to the well bore for production.
More recently, devices have been installed within the well bore, enabling control of the differential pressure via a fixed orifice type valve. As the pressure across the fixed orifice increases, so does the flow rate through the valve. These systems are commonly referred to as passive Inflow Control Devices (ICD).
More recently, valves have been introduced whereby the pressure differential can be varied via surface control, either via hydraulic conduits or via electrical lines, the control allowing adjustment of the orifice to vary the pressure drop across the valve and thereby alter the flow rate through the valve.
The complex nature of the flow path from the reservoir to the surface of a well and the many varying flow regimes which the fluid endure make it very difficult to predict the pressure drop from the formation to the well bore for production wells. Thus, it is difficult to predict in advance the size of the orifice of the valve to be installed or how to control valves from the surface in an optimal manner, which makes efficient reservoir production difficult.
Additionally, as the well produces with time, the heterogeneities in the formation and the changing conditions in the well will produce pressure variations between the formation and the well bore.